1. Field of the Invention
The present invention relates to an acoustic transducer that may be incorporated in a mobile cellular phone or other mobile communication devices to generate sound or vibration in response to an electric signal.
2. Description of the Background Art
Conventionally, mobile communication devices, e.g. cellular phones and PDAs, are arranged to inform the user of an incoming call by generating beep sound or melody sound, or by vibrating the enclosure of the device instead of producing sound when a “manner mode” has been selected. For this purpose, the conventional practice is to employ a structure incorporating a combination of a compact speaker and a vibrator that generates vibration by rotating an eccentric weight with a compact motor. Incorporating both the speaker and the vibrator as stated above, however, is disadvantageous from the viewpoint of achieving size and cost reduction of the device. Under these circumstances, a magnetically driven acoustic transducer capable of generating both sound and vibration by itself has been used recently.
FIG. 2 is a side view showing an example of such an acoustic transducer. The left-hand side of the figure is a sectional view showing the internal arrangement. The acoustic transducer has a tubular casing formed by joining together upper and lower casing segments 1 and 2 made of a plastic material. A protector 11 is secured to the top of the casing. Another protector 12 is secured to the bottom of the casing. The casing accommodates a diaphragm 9 and a magnetic exciter that vibrates the diaphragm 9. The magnetic exciter includes a voice coil 10 secured to the diaphragm 9 and a magnetic circuit part operatively (i.e. magnetically) connected to the voice coil 10. The magnetic circuit part includes annular lower and upper pole pieces 3 and 5 made of a magnetic material and an annular permanent magnet 4 sandwiched between the pole pieces 3 and 5. The lower pole piece 3 is provided with a cylindrical portion 3a extending upward from the inner peripheral edge thereof. A weight 6 is secured in the cylindrical portion 3a. The voice coil 10 extends into a magnetic gap g between the cylindrical portion 3a of the lower pole piece 3 and the upper pole piece 5.
An upper support spring 7 and a lower support spring 8 are secured to the top and bottom, respectively, of the magnetic circuit part. The respective outer peripheries of the two support springs 7 and 8 are secured to the casing for resiliently supporting the magnetic circuit part with respect to the casing. In connection with this, please refer to FIG. 1B in which a lower support spring 14 used in an acoustic transducer in accordance with the present invention is illustrated. As will be described in detail later, the lower support spring 14 comprises an inner annular portion 14b, an outer annular portion 14a and arcuate connecting portions 14c, 14c between the inner and outer annular portions. The support springs 7 and 8 of the prior art may be configured in the same shape as the lower support spring 14. The inner annular portion of the upper support spring 7 is joined to the upper side of the upper pole piece 5 by spot welding or the like. The inner annular portion of the lower support spring 8 is joined to the lower side of the lower pole piece 3.
Meanwhile, the outer annular portions of the support springs 7 and 8 are embedded and secured in the upper and lower casing segments 1 and 2, respectively, by insert molding. Thus, the magnetic circuit part is suspended by using two support springs, thereby allowing the magnetic circuit part to vibrate vertically without tilting, and thus preventing either the upper pole piece 5 or the lower pole piece 3 from contacting the voice coil 10, which would otherwise generate noise or cause breakage of components.
An example of the above-described double-sided suspension is found, for example, in Japanese Patent Application Publication (KOKAI) No. 2000-333282.
In the above-described acoustic transducer, when a driving signal of a certain frequency in an audio-frequency region is applied to the coil 10, the diaphragm 9, which has the voice coil 10 secured thereto, vibrates to generate sound, e.g. beep sound or voice. The magnetic circuit part supported by the support springs 7 and 8, however, does not substantially vibrate because its natural frequency is low. If the frequency of the driving signal is lower than the audio-frequency region, the vibration of the diaphragm 9 is so weak that no sound is generated, but instead the magnetic circuit part vibrates. The vibration of the magnetic circuit part is transmitted through the casing to the device incorporating the transducer, thus causing vibration of the device itself, but not making sound.
In the above-described acoustic transducer, the upper and lower casing segments 1 and 2 have the insert-molded support springs 7 and 8, respectively. The casing segments 1 and 2 are butt-joined together by ultrasonic welding and thus assembled into the casing. In the assembling process, it is difficult to join together the two casing segments 1 and 2 precisely so that the two support springs 7 and 8 are set parallel to each other. Consequently, the resonance frequency is likely to be displaced, causing the quality of the device to become unstable. The yield is also likely to be reduced.